Effect of a submicrosecond-advanced voltage pulse on the formation of a high-current electron beam in a magnetically insulated coaxial diode

The effect of a double accelerating voltage pulse on the formation of a high-current electron beam in a magnetically insulated coaxial diode under fore-vacuum pressures has been investigated. High voltage pulses of durations < 0.5 and similar to 1 ns and amplitudes (modulo) < 120 and >= 160 kV, respectively, were applied to a graphite cathode with a submicrosecond delay. When the time delay between the pulses was increased to a certain value at a residual gas pressure of 5 x 10(-3) Torr, the current of the second beam increased about fourfold and then decreased. The current reached a maximum of 4.5 kA, which was greater than that carried by the matched load of the high voltage generator. The observed effect can be accounted for by the neutralization of the beam charge in the plasma expanding from the cathode. The plasma channel could be formed due to explosive electron emission from the cathode or due to impact ionization of the residual gas.